Once air has been set in motion by the
pressure gradient force,
it undergoes an apparent
deflection from its path, as seen by an observer on the earth.
This apparent deflection is
called the "Coriolis force" and is a result of the earth's rotation.

As air moves from high to
low pressure in the northern hemisphere,
it is deflected to the right by the Coriolis force.
In the southern hemisphere, air moving from high to low pressure is
deflected to the left by the Coriolis force.

The amount of deflection the air makes is directly related to both the
speed at which the air is moving and its latitude.
Therefore, slowly blowing winds will be deflected only a small amount,
while stronger winds will be deflected more. Likewise, winds blowing
closer to the poles will be deflected more than winds at the same speed
closer to the equator. The Coriolis force is zero right at the equator.